1. Field of the Invention
This invention relates to methods of forming refractive index gratings in optical waveguides of particular, but not exclusive, application to forming such gratings in optical fibre waveguides.
2. Related Art
Optical waveguide gratings have many applications, for example as passive wavelength filters in wavelength multiplexed telecommunications and sensor systems and as frequency selective elements in active fibre devices.
One approach to making an optical waveguide grating is to form an external grating interacting with the evanescent field of the waveguide, for example by etching a grating close to the nearly exposed core of an optical fibre. A second approach, with which the present application is concerned, is to form a refractive index grating within the core of the waveguide. A standing wave is set up using two interfering beams derived from a single-frequency laser which, if sufficiently intense, writes a refractive index grating into the waveguide core over a time period in the order of minutes.
WO86/01303 published on the Feb. 27, 1986 describes a method of forming such a crating in an optical fibre waveguide by illuminating it transversely with a standing wave interference pattern set up by two suitable angled ultraviolet beams derived from a single coherent source. The two ultraviolet beams are produced by directing the source TV beam onto a beamsplitter which produces a pair of subsidiary beams which are reflected by a pair of mirrors to form a standing interference pattern in the region of the optical fibre. The grating spacing is controllable by varying the angle of incidence of the interfering subsidiary beams.
The writing wavelength is chosen to be one which efficiently modifies the refractive index of the medium. The grating so formed will be effective at longer wavelengths at which it may not be possible to write a grating.
This prior art apparatus for forming such gratings has several optical elements which must be maintained accurately in position relative to the UV source and optical fibre for the several minutes of exposure time needed to form the grating. This may be adequate when production of the gratings is made in carefully prepared and controlled surroundings. However, the applicant has identified a need for such gratings that necessitate the formation of gratings in less controlled surroundings.